1. Field of the Invention
The present invention relates to an improvement in image converter tubes: this improvement enables the elimination of the stray glimmer or glow that can develop on the insulators inside these tubes.
The invention also relates to a method implemented to eliminate this unwanted or stray glimmer.
A preliminary reminder of the structure and working of an image converter tube will provide for a clearer understanding of the nature of the problem posed and that of the solution proposed by the invention. However, more clearly and specifically, the explanations as well as those pertaining to the invention will be based, for example, on the non-restrictive example of a radiological image intensifier tube.
2. Description of the Prior Art
Image intensifier tubes are vacuum tubes comprising an input converter, placed in the front of the tube, an electronic optical system and a screen for the observation of the visible image placed in the rear of the tube, on the output window side of this tube.
In radiological image intensifier tubes (abbreviated as RII tubes), the input converter comprises a scintillator screen that converts the incident X photons into visible photons.
FIG. 1 shows a schematic view of a radiological type of image-intensifier tube such as this.
The RII tube comprises a glass or metal casing 1 of which one end, in front of the tube, includes an input screen 2. This end is closed by an input window 3 exposed to a radiation of X photons.
The second end of the casing forming the rear of the tube is closed by an output window 4 that is transparent to light.
The X-rays are converted into light rays by a scintillator screen 5. The light rays excite a photocathode 6 which produces electrons in response.
The electrons produced by the photocathode 6 are accelerated towards the output window 4 by means of different electrodes 7 and an anode 8, that is positioned along a longitudinal axis of the tube and forms the electronic optical system.
The output window 4 is formed by a transparent glass element which, in the example shown, bears a cathodoluminescent tube or output screen 9 formed by luminophors for examples.
The impact of the electrons on the cathodoluminescent screen or output screen enables the reconstitution of an image (amplified in luminance) which was initially formed on the surface of the photocathode 6.
The image displayed by the output screen 9 is visible through the glass element that constitutes the output window 4. Generally, optical sensor devices (not shown) are positioned outside the tube in the vicinity of the output tube 4 to pick up this image through the output window 4 and enable its observation.
However, this observation can be efficient only if no stray light comes into play. Now, one consequence firstly of the manufacturing method and, secondly, of the high voltages of the electronic optics, lies in the appearance of glimmer on the surface of the insulating parts that support the electrodes. It can easily be imagined that this glimmer lowers the quality of the radiological image observed, especially in terms of contrast.
This stray glimmer arises out of the fact that the quality of the electrical insulation of the electrodes is lowered by the presence of the alkaline metals that are deposited on the electrodes and which, by field effect, foster an emission of electrons that will charge the insulators.